Low temperature probe and nuclear magnetic resonance analysis apparatus using the same

ABSTRACT

In an ultrahigh sensitive NMR apparatus, a stability of a temperature of a very low temperature probe coil is improved. A low temperature probe of the high sensitive NMR apparatus is structured at a very low temperature by a cooling apparatus. A cooling medium at a room temperature (300 K) discharged from a compressor ( 1 ) is cooled down to 70 K by a countercurrent heat exchanger ( 2 ), and is next cooled down to 4 K or less by a series of second stages ( 7, 8 ). Further, the cooling medium enters into a low temperature probe ( 15 ) via a transfer tube ( 9 ), cools a probe coil ( 11 ) to 5 to 10 K by a heat exchanging portion ( 10 ), further cools a radiation shield ( 13 ) to 40 to 60 K, and makes a circuit via the countercurrent heat exchanger ( 2 ). A high temperature stability can be applied to a receiving probe coil in the low temperature probe by a cooling apparatus having a high temperature stability.

TECHNICAL FIELD

The present invention relates to a low temperature probe for a nuclearmagnetic resonance (NMR), and a nuclear magnetic resonance analysisapparatus using the same.

BACKGROUND ART

The nuclear magnetic resonance apparatus corresponds to an apparatus forirradiating an electromagnetic wave in a state of arranging a samplebelow a static magnetic field, observing and analyzing a freeattenuation signal from an atomic nucleus in the sample generatedthereby, and executing a structural analysis of a material in thesample. In recent years, it is expected to be applied particularly to anorganic polymer such as a protein or the like.

The conventional NMR apparatus is structured such that a superconductingmagnet is arranged perpendicular to an installation surface, and a probeis inserted to the superconducting magnet from a vertical direction tothe installation surface. On the other hand, a high sensitive NMRapparatus obtained by applying an improvement to the conventional NMRapparatus is described in patent documents 1 and 2 (JP-A-2003-329755 andJP-A-2003-329756). The high sensitive NMR apparatus is provided with apair of split superconducting magnets arranged in a horizontal directionin the installation surface, and has a low temperature probe inserted tothe split superconducting magnet from a horizontal direction andincluding a transmitting probe coil and a receiving probe coil.

In order to obtain a high sensitivity in the NMR apparatus, it iseffective to cool the receiving probe coil to a very low temperature. Inorder to operate stably for a long time, there can be considered amethod of cooling the receiving probe coil by circulating a coolingmedium, however, a cooling performance of a refrigerating machine has alimit. Further, it is hard to obtain a temperature stability withrespect to a heat generated in the transmitting probe coil at a time ofirradiating a radio wave. In addition, since both of a specific heat anda coefficient of thermal conductivity become lower in most materialsunder a very low temperature state, it becomes further hard to obtain ahigh-level temperature-stability.

In order to obtain a temperature stability, there is a method utilizinga latent heat of a helium, however, a high cooling performance isrequired in the refrigerating machine, and a refrigerating machineutilizing a Joule-Thomson effect is frequently used. This kind ofrefrigerating machine having a high cooling performance is hard to behandled. A cooling apparatus using a general Gifford McMahonrefrigerating machine (a GM refrigerating machine) rather has anadvantage for a user although a cooling performance is inferior to theabove.

Patent document 3 (JP-A-10-332801) describes a structure of an NMR probewhich is operated at a very low temperature while using the coolingapparatus such as the GM refrigerating machine. However, in the casethat the cooling apparatus is structured by one GM refrigerating machineand two or more countercurrent heat exchangers, it is hard to supply acooling medium at a stable temperature with respect to a loadfluctuation of the refrigerating machine. The load fluctuation in thiscase is constituted by an electric heat generation of the transmittingprobe coil. If no countermeasure is applied, the temperature of theprobe coil fluctuates at a certain cycle and a time constant.

In the structure of the single turn superconducting magnet and the lowtemperature probe as described in the patent document 3, the sensitivityis saturated together with a reduction of the temperature, and it isimpossible to obtain a benefit caused by an improvement of thesensitivity on the basis of the cooling at a certain temperature orless. Accordingly, the receiving probe coil is cooled at a temperaturein the vicinity of 20 K. On the other hand, in the low temperature probehaving the split type superconducting magnet and the solenoid typereceiving probe coil described in the patent documents 1 and 2, thesensitivity is described as being improved to a lower temperature.Therefore, the receiving probe coil is cooled in the vicinity of 5 K.

FIG. 9 shows a comparison between the temperature and the sensitivity inthe solenoid type and split type probe coils. As illustrated, asensitivity (S/N) of the split type is high in the low temperatureregion.

The probe of the NMR apparatus is sensitive to a fluctuation of thestate. With respect to a room temperature and a temperature of a sample,a fluctuation width of 0.01 or a level in proportion thereto is oftenrequired. In the high sensitive NMR apparatus described in the patentdocuments 1 and 2, a temperature stability of the receiving probe coilis particularly important.

In the case of cooling the receiving probe coil in the very lowtemperature state by one of low temperature side stages of therefrigerating machine, if a temperature of a part of the circulatingcooling medium is increased due to a heat generation of the transmittingprobe coil or the like, it takes a long time to average a temperature ofan entire of the circulating cooling medium. The cooling mediumexchanging heat with the receiving probe coil has a time change with anexternal waviness. This phenomenon is not negligible in the case of thehigh sensitive measurement even in the conventional NMR apparatus.Further, in the ultrahigh sensitive NMR apparatus described in thepatent documents 1 and 2, it is not allowable.

In the case that the conventional cooling apparatus is applied as it isto the low temperature probe of the ultrahigh sensitive NMR apparatus,the receiving probe coil has a risk in the temperature stability asmentioned above, and the reduction of the sensitivity caused thereby iscalculated.

The temperature change of the probe coil and a resonance circuit portionforming a pair therewith causes a change in a circuit constant or thelike, and exerts an influence on an acquired signal. In particular,there are generated a change in a resistance of a wiring on the basis ofthe temperature change, and a change in a capacity of a condenser.Further, there are generated a change in a resonance frequency, a changein a Q value, and a change in an input impedance.

In order to structure the high sensitive NMR apparatus, it is preferablethat the Q value of the resonance is higher. The higher a purity of amaterial is, the more the material is affected by the temperaturechange. In the case that a superconductive material is used, theproperty is sensitively changed by the temperature. In addition, a veryuniform magnetic field is required in the NMR apparatus, however, sincea magnetism of the constituting material is changed by the temperatureat the very low temperature, there is generated a problem relating to amagnetic homogeneity.

SUMMARY OF THE INVENTION

The present invention is made by taking the problem in the prior artmentioned above into consideration, and an object of the presentinvention is to provide a low temperature probe applying a hightemperature stability to a receiving probe coil of the low temperatureprobe by using a cooling apparatus having a high temperature stability,for the purpose of bringing out a performance of an ultrahigh sensitiveNMR apparatus, and a nuclear magnetic resonance analysis apparatus witha high sensitivity using the same.

In order to achieve the object mentioned above, in accordance with thepresent invention, there is provided a low temperature probe having atransmitting coil and a receiving coil or a transmit/receive coil, andused in an NMR apparatus, comprising:

an opposed head exchanger (called as a countercurrent heat exchanger inan embodiment) cooling a cooling medium at a room temperature from acompressor to 70 K or less in one side;

a cooling apparatus structured by connecting at least two refrigeratingmachines having two stages in series, and cooling the cooling mediumfrom the opposed heat exchanger, the two cooling stages having a firstcooling stage capable of cooling to 30 K or less and a second coolingstage capable of cooling to 4 K or less;

a probe portion having a first heat exchanging portion executing a heatexchange between the cooling medium from the cooling apparatus and thereceiving coil or the transmit/receive coil; and

a circulation structure circulating the cooling medium from the probeportion into the other side of the opposed heat exchanger.

Further, the structure is made such that the probe portion is providedwith a second heat exchanging portion executing a heat exchange betweenthe cooling medium from the first heat exchanging portion and a maskingshield internally wrapping the cooling medium from the first heatexchanging portion and the receiving coil or the transmit/receive coil,and the cooling medium from the second heat exchanging portion iscirculated into the other side of the opposed heat exchanger.

Further, the structure is made such that the opposed heat exchanger andthe cooling apparatus are arranged in a vacuum tank, an outward routeand a homeward route of the first heat exchanger and the second heatexchanger are connected by a pair of cooling medium transport paths.

Further, the structure is made such that a buffer tank having a largercapacity than the cooling medium flowing through the circulationstructure is provided between an outlet of the compressor and an inletof the opposed heat exchanger.

Further, the structure is made such that a piping diameter is madenarrower toward a rear stage in a piping between the compressor and theopposed heat exchanger, a piping in the vicinity of the first coolingmedium stage, a piping between the second cooling medium stages and apiping after the cooling medium stage.

Further, the structure is made such that the diameter of the pipingbetween the second stages and after the piping is set about one third ofthe diameter of the piping in the vicinity of the first stage, and alength of the piping is set to about 1.7 times.

In accordance with the present invention, there is provided a nuclearmagnetic resonance analysis apparatus comprising:

a split type superconducting coil; and

a probe for an NMR inserted to the split,

wherein the low temperature probe used in the NMR apparatus mentionedabove is used in the NMR probe.

In accordance with the present invention, two or more GN refrigeratingmachines are used for the circulating cooling system capable ofsupplying the cooling medium having the very low temperature, a coolingorder of the circulating cooling medium is constituted by a serialrefrigerating machine connection, and the transmitting and receivingprobe coil is cooled by a pair of reciprocating cooling medium transportpaths (transfer tubes) and a radiant shield.

As mentioned above, in accordance with the present invention, it ispossible to provide the low temperature probe in which the lowtemperature stability is high, by using the simple GM refrigeratingmachine. Further, it is possible to achieve the nuclear magneticresonance analysis apparatus having the high sensitivity by using thelow temperature probe.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a low temperature probe in accordance withan embodiment 1 of the present invention;

FIG. 2 is a schematic view of a high sensitive NMR apparatus;

FIG. 3 is a view of a schematic structure of an integral type probe;

FIG. 4 is a view of a coil structure of a separate type probe;

FIG. 5 is a characteristic view showing a capacity curve of a typical GMrefrigerating machine;

FIG. 6 is a view of a schematic structure of a countercurrent heatexchanger;

FIG. 7 is a schematic view of a low temperature probe provided with abuffer tank in the embodiment 1;

FIG. 8 is a schematic view of a low temperature probe in accordance withan embodiment 2 of the present invention; and

FIG. 9 is a characteristic view showing a comparison between atemperature and a sensitivity of a probe coil in an NMR apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will be in detail given below of a plurality ofembodiments in accordance with the present invention with reference tothe accompanying drawings.

Embodiment 1

A nuclear magnetic resonance analysis (NMR) apparatus corresponds to anapparatus for irradiating an electromagnetic wave in a state ofarranging a sample below a static magnetic field, observing andanalyzing a free attenuation signal from an atomic nucleus in the samplegenerated thereby, and executing a structural analysis of a material inthe sample. In recent years, it is expected to be applied particularlyto an organic polymer such as a protein or the like.

In general, the NMR apparatus has a cylindrical superconducting magnetfor generating a static magnetic field, and a probe constituted by atransmitting probe coil serving as a means for irradiating anelectromagnetic wave with respect to a sample, and a receiving probecoil receiving a free attenuation signal. Further, the NMR apparatus isstructured such as to be connected to the transmitting probe coil andthe receiving probe coil and be provided with a measuring apparatusexecuting a measurement and an analysis of the signal.

FIG. 2 shows a schematic view of an ultrahigh sensitive NMR apparatus inaccordance with an embodiment 1. A superconducting magnet 14 isstructured as a pair of split type, and has a low temperature probehaving a transmitting probe coil and a receiving probe coil inserted toa split superconducting magnet. In FIG. 2A, a probe 15 is inserted in avertical direction from a lower side of the magnet 14, and the receivingprobe coil is arranged at a center of a uniform magnetic field formed bythe superconducting magnet. In FIG. 2B, the probe 15 is inserted in ahorizontal direction from a lateral side of the magnet 14, and thereceiving probe coil is arranged at the center of the uniform magneticfield formed by the superconducting magnet.

Since it is necessary to cool down the receiving probe coil 11 to a verylow temperature equal to or less than an approximately 20 K, a coolingapparatus 17 is attached to the probe 15 via a cooling medium transportpath 9.

The receiving probe coil 11 is attached to a portion near a leading endof the probe 15, and is cooled down on the basis of a heat exchange witha cooling medium transported through a piping within the probe body 16.In the NMR apparatus aiming at an ultrahigh sensitivity, an operationtemperature is required a very low temperature equal to or less than anapproximately 10 K, desirably about 5 K. This means that a highsignal-noise ratio (S/N) is obtained by lowering a thermal noise in thereceiving probe coil 11 and an electric circuit portion. Further, in thecase of the receiving coil using a superconducting pair, there isconsidered that a lower temperature can improve a characteristic of asuperconductor.

FIG. 3 shows an internal structure of the low temperature probe. Aninner portion of the low temperature probe 15 is kept in a high vacuumstate, and is vacuum insulated. A transmit/receive probe coil 11 isarranged near a leading end of a narrow and long cylinder inserted tothe split of the superconducting magnet 14. A reciprocating gas supplypipe line transporting the cooling medium is extended near thetransmit/receive probe coil. The cooling medium and the transmit/receiveprobe 11 are heat exchanged with each other via a heat exchanger 10, andthe transmit/receive probe coil 11 is cooled. Further, in order to keepthe inner portion at the very low temperature, a heat radiation shield13 surrounds the receiving probe coil 11, the heat exchanger 10 and thepipe line. The heat radiation shield 13 exchanges heat with a heatexchanger 12 in a return side of the pipe line, and is cooled down.

FIG. 3 shows a structure in which the low temperature probe is insertedin the horizontal direction to the static magnetic field in thehorizontal direction generated by the horizontal split typesuperconducting magnet, such as FIG. 2B. The structure is made such thatthe inserting direction of the probe and a direction of the magneticfield detected by the receiving probe coil 11 are orthogonal. On theother hand, in the case of FIG. 2A, the structure is made such that theinserting direction of the probe and the direction of the magnetic fielddetected by the receiving probe coil coincide with each other.

In the probe in FIG. 3, the transmit/receive probe coil is structuredsuch that the receiving probe coil and the transmitting probe coil areintegrated, and executes the transmit and receive by one probe coil. Onthe other hand, there is a case that the receiving probe coil and thetransmitting probe coil are separated.

FIG. 4 shows a structure in which the receiving probe coil and thetransmitting probe coil are separated. As illustrated, a solenoid typecoil 111 and a saddle type coil 112 are provided, and two coils arearranged such that an electromagnetic interaction becomes as small aspossible. In this case, the solenoid type coil 111 in the inner sidecorresponds to the receiving probe coil employing the superconductingmaterial, and the saddle type coil 112 in the outer side corresponds tothe transmitting probe coil employing a normal conducting material. Inthis case, even if roles of the transmitting probe coil and thereceiving probe coil are replaced, it is possible to measure, and it ispossible to employ a structure in which the transmit and receive areintegrated.

A description will be given below of a probe cooling system which isrequired for the ultrahigh sensitive NMR apparatus, and can obtain ahigh temperature stability of the receiving probe coil.

FIG. 1 shows a schematic view of the low temperature probe in accordancewith the embodiment 1 and a cooling apparatus thereof. In this case, aportion relating to a cooling structure is shown, and a description willbe given of a case that two refrigerating machines are provided. Even ifthe number of the refrigerating machines is increased to three or four,the same principle is applied. It goes without saying that the probe ofthe NMR apparatus requires a resonance circuit with the transmit/receiveprobe coil, a high frequency cable, a preamplifier an adjustingmechanism for a resonance frequency and the like, in addition to theillustrated structure.

The refrigerating machines 3 and 4 used in the present embodiment areconstituted by a two-stage GM refrigerating machine, and are structuredsuch that an ultimate temperature of the first stages 5 and 6 is equalto or less than 25 K, an ultimate temperature of the second stages 7 and8 is equal to or less than 3 K, and a refrigerating capacity at 4 K isabout 1 W. The GM refrigerating machine corresponds to a refrigeratingmachine in which a helium gas is used as a cooling medium, a coolingoperation is executed by repeating a compression and an expansion of thecooling medium, and a cool storage medium is arranged in a lowtemperature portion. It has a cooling capacity of about 1 W at atemperature of about 4 K, and has a great advantage that a very lowtemperature can be easily generated by pushing a button.

FIGS. 5A and 5B show a cooling capacity curve of a typical refrigeratingmachine. FIG. 5A shows a cooling capacity of a first stage, and FIG. 5Bshows a cooling capacity of a second stage. The lower a stagetemperature of the GM refrigerating machine is, the lower the coolingcapacity is. Further, the second stage can cool to a lower temperature.

The cooling apparatus in accordance with the present embodimentcirculates the cooling medium at a room temperature by a compressor 1.The compressor 1 has an intake port and a discharge port, compresses agas taken from the intake port in an inner portion of the apparatus, anddischarges the compression gas from the discharge port. Since a pressuredifference is generated by the discharge port and the intake port, it ispossible to circulate the gas within the piping. Since it is necessaryto cool particularly the receiving probe coil 11 to a very lowtemperature, a helium gas is suitable for the circulating gas coolingmedium. A description will be given below in the order of a circulatingroute.

The helium gas discharged from the compressor 1 and introduced into arefrigerating apparatus (a vacuum tank) from the room temperaturethrough the piping is heat exchanged by the countercurrent heatexchanger 2. An inner side of the refrigerating apparatus is kept in avacuum state for a heat insulation.

FIG. 6 is a schematic view showing a principle of the countercurrentheat exchanger. One side port of two adjacent flow paths forms a hightemperature region, an opposite side port forms a low temperatureregion, and it is possible to execute a heat exchange between the flowpaths. The helium gas introduced from the lower temperature sidereceives the heat, and comes closer to the temperature in the hightemperature side so as to go out from the heat exchanger. On thecontrary, the helium gas introduced from the high temperature side comescloser to the temperature in the low temperature side so as to go outfrom the heat exchanger.

The countercurrent heat exchanger 2 can form a cooling apparatus havinga good efficiency by being inserted between the room temperature and thelow temperature, between an intermediate temperature and a very lowtemperature and between regions having largely different temperatures. Atemperature TH1 of the helium gas from the high temperature side of thecountercurrent heat exchanger 2 is about 300 K corresponding to the roomtemperature, and a temperature TL2 of the helium gas moving forward fromthe lower temperature side is about 60 K. A temperature TL1 of the gasintroduced from the room temperature is cooled to about 70 K byexecuting the heat exchange and an efficient circulation is executed.

The helium gas heat exchanged by the countercurrent heat exchanger 2 soas to be cooled leaves for the first cooling stage 5 of the firstrefrigerating machine 3. The temperature of the helium gas after theheat exchange by the countercurrent heat exchanger 2 is about 70 K, andan outlet temperature of the helium gas in the first cooling stage ofthe refrigerating machine 3 becomes about 40 K. Next, the outlettemperature of the first stage becomes about 25 K toward the firstcooling stage 6 of the second refrigerating machine 4.

The temperature formed in the first stage is limited to 25 to 30 K inthe general GM refrigerating machine. In order to cool to the lowertemperature, it is necessary to cool by using the second stage as in thepresent embodiment.

The helium gas cooled to 70 K to 25 K by the first stage of therefrigerating machines 3 and 4 next leaves for the second stage 7 of thefirst refrigerating machine 3. In this case, the helium gas is cooled toabout 8 K by the second stage 7 of the refrigerating machine 1, furtherleaves for the second stage 8 of the second refrigerating machine 4, andis cooled to about 4 K by the second stage 8.

In the following description, a description “series connection ofrefrigerating machines” means a connecting way in which the helium gascirculates through the first stage in the order of the refrigeratingmachines, and thereafter circulates through the second stage in theorder of the refrigerating machines. The connection order does notchange even in the case that the number of the refrigerating machines isincreased.

As mentioned above, the helium is cooled to 4 K from about 70 K, and isintroduced to the heat exchanger 10 with the transmit/receive probe coil11 through the transfer tube 9. The heat exchanger 10 and thetransmit/receive probe coil 11 are thermally connected, and the probecoil is cooled on the basis of a conduction. In this case, FIG. 1 showsonly the receiving probe coil in the transmit/receive probe coil 11,however, the transmit/receive probe coil 11 is actually constituted byboth the transmitting probe coil and the receiving probe coil as shownin FIG. 4.

It goes without saying that the structure can be made such that thetransmit and receive are integrated. However, it is hard to employ thecharacteristic of the superconducting material under the high magneticfield as the transmitting probe coil or the high sensitive probe coilcapable of transmitting and receiving. In other words, the transmittingprobe coil generates a heat generation due to an electric factor, andthe heat generation is largely changed in time due to a pulse of a radiowave. In particular, there exist a case that the heat generation isconcentrated during some micro second, and a case that the heatgeneration is never generated.

Accordingly, the temperature of the helium ascends to about 5 to 10 K onthe basis of the degree of the heat generation of the transmitting probecoil. Further, in order to efficiently execute the heat exchange, it isnecessary to make a heat resistance between the receiving probe coil andthe heat exchanger 10 as small as possible.

The transmit/receive probe coil is cooled on the basis of theconduction, however, the conduction portion can use a material having ahigh electric resistance and a high coefficient of thermal conductivity,for example, an aluminum nitride or a sapphire. If a material having agood conductivity is used, it is very hard to execute a normalmeasurement due to a noise by an eddy current. Accordingly, such amaterial can not be used. Further, in order to make the thermalresistance small, it is preferable to make a distance between the heatexchanger and the transmit/receive probe coil short so as to secure anarea efficient for the heat transfer. It is possible to effectively coolthe transmit/receive probe coil on the basis of the device mentionedabove.

The helium warmed up by the probe coil 11 enters into the heat exchanger12, and is utilized for cooling the radiation shield 13. The radiationshield 13 is cooled to about 40 to 60 K. In this case, the radiationshield 13 is arranged so as to surround the very low temperature regionincluding the receiving probe coil 11, and serves to shield anapproaching heat due to a radiation, and inhibit the approaching heat inthe low temperature side.

The radiation shield 13 keeps the helium temperature after cooling theradiation shield approximately constant, in addition to the originalrole of shielding the heat intrusion on the basis of the radiation. Thisis because a heat capacity of the radiation shield 13 is sufficientlylarge in comparison with a heat capacity of the circulating helium.Accordingly, the temperature of the helium returned to thecountercurrent heat exchanger 2 through the return path of the transfertube 9 is kept approximately constant, and the helium gas which is againheat exchanged from the room temperature is always kept approximately ata fixed temperature. The temperature entering into the refrigeratingmachine is always approximately fixed, and the temperature after beingcooled has a high stability. In this case, if the heat capacity of theradiation shield 13 is made too large, an effect obtained thereby issmall, and an initial cooling time for cooling to a steady state fromthe room temperature only becomes long. As a material of the radiationshield 13, a material having a good heat conductivity, for example, anoxygen free copper, a high purity aluminum or the like is suitable.

The description is given of a matter that the helium gas circulatingbetween the cooling apparatus and the probe is cooled to about 4 Kwithin the cooling apparatus by connecting the pipes in the respectivecooling stages 5 to 8 of the refrigerating machines 3 and 4 in series. Adescription will be further given below of details thereof.

A loss in the serial countercurrent heat exchanger 2 depends on a massflow rate of the cooling medium. For example, in the countercurrent heatexchanger having an efficiency of about 0.95, a loss about 5 W isgenerated with respect to a flow rate of 0.1 l gram per secondcorresponding to an expected flow rate. A higher heat exchangeefficiency is preferable in the countercurrent heat exchanger, however,too large size is disadvantageous in view of an entire of the apparatus.In the case that the flow rate of the cooling medium is set to 0.1 lgram per second, a cooling amount in the refrigerating machine is about40 W in total of the cooling stages, however, a cooling capacityeffective for cooling the low temperature probe becomes about 35 Wobtained by subtracting the loss 5 W mentioned above therefrom.

Accordingly, it is necessary to set a heat load of the entire lowtemperature probe including the cooling apparatus within the value. Theheat load allowed by the calculation corresponds to a calorie making anintrusion into the radiation shield from the vacuum container at theroom temperature, and is determined by a sum of the heat radiation andthe calorie entering from the support portion of the shield.

In order to inhibit the heat intrusion on the basis of the heatradiation, it is effective to use a layered heat insulating material.The layered heat insulating material is structured by forming a metalfilm on a surface of a thin heat insulating sheet, and is used by beingalternately lapped over a heat insulating spacer. The heat radiationshield is fixed and supported by using a heat insulating material suchas an FRP or the like in such a manner that a heat entering amountbecomes sufficiently small. The specification mentioned above can beachieved by designing while sufficiently paying an attention to the heatinsulating property, as mentioned above.

Next, a description will be given of a flow rate of the helium gas forefficiently cooling. If the flow rate is made too large due to the lossof the countercurrent heat exchanger, it is impossible to make thetemperature of the cooling medium sufficiently low. Further, on theother hand, if the flow rate is made too small, the temperature increaseduring the transport become large. For an efficient operation, it isnecessary to operate in a state of setting a suitable flow rate.

An optimum flow rate is changed on the basis of a balance between thecooling capacity of the refrigerating machine and the heat load of theentire of the low temperature probe, however, is considered to existbetween 0.05 and 0.5 gram per second in accordance with an estimation onthe basis of the efficiency of the heat exchanger and the capacity ofthe refrigerating machine. It goes without saying that the cooling canbe more efficiently executed in accordance that the heat load of theentire of the low temperature probe is smaller, and the efficiency ofthe countercurrent heat exchanger 2 is higher.

A specific heat of the cooling medium at the very low temperaturecorresponds to an important factor for obtaining a stability. In thecase that the helium gas is employed as the cooling medium, the specificheat is between 4 and 5 K and becomes comparatively high. Accordingly,it is preferable to operate at a pressure between about 0.25 and 0.5MPa. In order to keep the pressure stably, it is effective to connect agas having a sufficiently larger than the amount of the gas circulatingwithin the piping, as a buffer, to the piping at the room temperature.

FIG. 7 shows a structure of a cooling system having a buffer tank. Incomparison with the system in FIG. 1, a buffer tank 17 is provided, anda sufficiently larger amount of gas than the amount of the gascirculating within the piping is charged.

The refrigerating capacity itself is increased by using two or morerefrigerating machines. In the present embodiment, since the coolingstages of two refrigerating machines are connected in series, the timevarying heat load generated in the transmitting probe coil is taken onfour cooling stages. Accordingly, it is possible to obtain a highertemperature stability than the case that the heat load is taken on onlyone cooling stage.

The cooling medium supply temperature is substantially determined by atemperature of the cooling stage in the final stage. A rate of the heatload fluctuation actually taken on the cooling stage in the final stagecan be considered as (cooling capacity in final stage of refrigeratingmachine)/(cooling capacity of entire cooling stage). Accordingly, itbecomes 1/40 of the heat load fluctuation in the case of being taken onone cooling stage, and it is possible to significantly inhibit thetemperature fluctuation. For example, in the case that the temperatureof the cooling medium gas before cooling becomes 1 K higher, thetemperature of the cooling medium after cooling changes about 1/40 K.

As mentioned above, a multistage refrigerating machine is structured byconnecting a plurality of refrigerating machines in series so as toincrease a cooling capacity, and the fluctuation of the heat load isdispersed. Further, the cooling medium having a high temperaturestability is supplied to the receiving probe coil by connecting the heatexchanger of the receiving probe coil and the heat exchanger of theradiation shield in series, and using the radiation shield as a thermalbuffer.

It is possible to achieve the low temperature probe having a highstability on the basis of the provision of the cooling mechanismmentioned above. Further, it is possible to achieve the NMR apparatussuitable for the high sensitive measurement having a high stability, byusing the low temperature probe in accordance with the presentembodiment.

Embodiment 2

The cooling apparatus and the cooling mechanism of the low temperatureprobe described in the embodiment 1 do not particularly refer to thepiping of the cooling medium. It is possible to further improve thecooling efficiency and the stability by devising a cross sectional shapeof the piping.

FIG. 8 shows a cooling structure of a probe in accordance with anembodiment 2. A basic structure is not different from the embodiment 1in FIG. 1, however, piping diameters of a pipe 18 at the roomtemperature, front and rear pipes 19 cooled in the first stages 5 and 6,and a pipe 20 after being cooled in the second stages 7 and 8 arechanged. As illustrated, the pipe 18, the pipe 19 and the pipe 20 havedifferent thicknesses, and the thickness of the pipe is made narrower inaccordance with the lower temperature. The pipe 18 in the region of theroom temperature is preferably structured such that the pressure loss issufficiently smaller with respect to the circulating gas amount.

In the case that the helium is used as the cooling medium for cooling tothe very low temperature, a volume at the room temperature and a volumeat the low temperature are largely changed. In particular, a density ofthe helium at 0.5 MPa and 300 K is 0.8 kg/m3. On the contrary, it is2.39 kg/m3 at 100 K and is 129 kg/m3 at 5 K. Since the mass flow amountbecomes constant within the piping in a steady state in the case ofconnecting in series, a volume flow rate and a Reynolds number relatingthereto are reduced in the low temperature portion. An efficiency of theheat exchanger is affected by a surface area of the flow path and a flowspeed of the fluid, that is, the Reynolds number, and an efficiency ofthe heat exchange with the cooling stage is lowered in the second stageside in which the Reynolds number becomes lower, in comparison with thefirst stage side.

It is possible to execute the cooling operation without deterioratingthe efficiency by reducing the flow path cross sectional area of thepipe in accordance with the cooling from 300 K, making the crosssectional area in the second stage portion about ⅓ in the first stage,and making the length thereof about 1.7 times, as in the presentembodiment.

Actually, there is a method of structuring simpler and connecting thepipes having different thicknesses before or after the cooling head ofthe refrigerating machine. This method can make the designs of the heatexchangers in two stages common, and is advantageous in view of a costin comparison with the case that two kinds of heat exchangers havinglargely different structures are prepared.

The lower the temperature is, the larger the change of the volume on thebasis of the temperature change is. If the volume of the very lowtemperature region is set small, a variation of the change amount of thevolume on the basis of a slight change of the temperature becomes small,and the pressure is hardly changed. As a result, the temperature isstabilized. The effect of making the thickness of the pipe smaller inaccordance with the lower temperature further improves the effectapplied to the stability of the buffer tank mentioned in the embodiment1.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A low temperature probe having a transmitting coil and a receivingcoil or a transmit/receive coil, and used in an NMR apparatus,comprising: an opposed head exchanger (called as a countercurrent heatexchanger in an embodiment) cooling a cooling medium at a roomtemperature from a compressor to 70 K or less in one side; a coolingapparatus structured by connecting at least two refrigerating machineshaving two stages in series, and cooling the cooling medium from saidopposed heat exchanger, said two cooling stages having a first coolingstage capable of cooling to 30 K or less and a second cooling stagecapable of cooling to 4 K or less; a probe portion having a first heatexchanging portion executing a heat exchange between the cooling mediumfrom said cooling apparatus and said receiving coil or saidtransmit/receive coil; and a circulation structure circulating thecooling medium from said probe portion into the other side of saidopposed heat exchanger.
 2. A low temperature probe for an NMR apparatusas claimed in claim 1, wherein said probe portion is provided with asecond heat exchanging portion executing a heat exchange between thecooling medium from said first heat exchanging portion and a maskingshield internally wrapping the cooling medium from said first heatexchanging portion and said receiving coil or said transmit/receivecoil, and the cooling medium from said second heat exchanging portion iscirculated into the other side of said opposed heat exchanger.
 3. A lowtemperature probe for an NMR apparatus as claimed in claim 2, whereinsaid opposed heat exchanger and said cooling apparatus are arranged in avacuum tank, an outward route and a homeward route of said first heatexchanger and said second heat exchanger are connected by a pair ofcooling medium transport paths.
 4. A low temperature probe for an NMRapparatus as claimed in claim 2, wherein a buffer tank having a largercapacity than the cooling medium flowing through said circulationstructure is provided between an outlet of said compressor and an inletof said opposed heat exchanger.
 5. A low temperature probe for an NMRapparatus as claimed in claim 3, wherein a buffer tank having a largercapacity than the cooling medium flowing through said circulationstructure is provided between an outlet of said compressor and an inletof said opposed heat exchanger.
 6. A low temperature probe for an NMRapparatus as claimed in claim 2, wherein a piping diameter is madenarrower toward a rear stage in a piping between said compressor andsaid opposed heat exchanger, a piping in the vicinity of said firstcooling medium stage, a piping between said second cooling medium stagesand a piping after said cooling medium stage.
 7. A low temperature probefor an NMR apparatus as claimed in claim 3, wherein a piping diameter ismade narrower toward a rear stage in a piping between said compressorand said opposed heat exchanger, a piping in the vicinity of said firstcooling medium stage, a piping between said second cooling medium stagesand a piping after said cooling medium stage.
 8. A low temperature probefor an NMR apparatus as claimed in claim 4, wherein a piping diameter ismade narrower toward a rear stage in a piping between said compressorand said opposed heat exchanger, a piping in the vicinity of said firstcooling medium stage, a piping between said second cooling medium stagesand a piping after said cooling medium stage.
 9. A low temperature probefor an NMR apparatus as claimed in claim 5, wherein a piping diameter ismade narrower toward a rear stage in a piping between said compressorand said opposed heat exchanger, a piping in the vicinity of said firstcooling medium stage, a piping between said second cooling medium stagesand a piping after said cooling medium stage.
 10. A low temperatureprobe for an NMR apparatus as claimed in claim 6, wherein the diameterof the piping between said second stages and after the piping is setabout one third of the diameter of the piping in the vicinity of saidfirst stage, and a length of the piping is set to about 1.7 times.
 11. Alow temperature probe for an NMR apparatus as claimed in claim 7,wherein the diameter of the piping between said second stages and afterthe piping is set about one third of the diameter of the piping in thevicinity of said first stage, and a length of the piping is set to about1.7 times.
 12. A low temperature probe for an NMR apparatus as claimedin claim 8, wherein the diameter of the piping between said secondstages and after the piping is set about one third of the diameter ofthe piping in the vicinity of said first stage, and a length of thepiping is set to about 1.7 times.
 13. A low temperature probe for an NMRapparatus as claimed in claim 9, wherein the diameter of the pipingbetween said second stages and after the piping is set about one thirdof the diameter of the piping in the vicinity of said first stage, and alength of the piping is set to about 1.7 times.
 14. A nuclear magneticresonance analysis apparatus comprising: a split type superconductingcoil; and a probe for an NMR inserted to the split, wherein the lowtemperature probe used in the NMR apparatus as claimed in claim 1 isused in said NMR probe.
 15. A nuclear magnetic resonance analysisapparatus comprising: a split type superconducting coil; and a probe foran NMR inserted to the split, wherein the low temperature probe used inthe NMR apparatus as claimed in claim 2 is used in said NMR probe.
 16. Anuclear magnetic resonance analysis apparatus comprising: a split typesuperconducting coil; and a probe for an NMR inserted to the split,wherein the low temperature probe used in the NMR apparatus as claimedin claim 3 is used in said NMR probe.
 17. A nuclear magnetic resonanceanalysis apparatus comprising: a split type superconducting coil; and aprobe for an NMR inserted to the split, wherein the low temperatureprobe used in the NMR apparatus as claimed in claim 4 is used in saidNMR probe.
 18. A nuclear magnetic resonance analysis apparatuscomprising: a split type superconducting coil; and a probe for an NMRinserted to the split, wherein the low temperature probe used in the NMRapparatus as claimed in claim 5 is used in said NMR probe.
 19. A nuclearmagnetic resonance analysis apparatus comprising: a split typesuperconducting coil; and a probe for an NMR inserted to the split,wherein the low temperature probe used in the NMR apparatus as claimedin claim 6 is used in said NMR probe.
 20. A nuclear magnetic resonanceanalysis apparatus comprising: a split type superconducting coil; and aprobe for an NMR inserted to the split, wherein the low temperatureprobe used in the NMR apparatus as claimed in claim 7 is used in saidNMR probe.
 21. A nuclear magnetic resonance analysis apparatuscomprising: a split type superconducting coil; and a probe for an NMRinserted to the split, wherein the low temperature probe used in the NMRapparatus as claimed in claim 8 is used in said NMR probe.
 22. A nuclearmagnetic resonance analysis apparatus comprising: a split typesuperconducting coil; and a probe for an NMR inserted to the split,wherein the low temperature probe used in the NMR apparatus as claimedin claim 9 is used in said NMR probe.
 23. A nuclear magnetic resonanceanalysis apparatus comprising: a split type superconducting coil; and aprobe for an NMR inserted to the split, wherein the low temperatureprobe used in the NMR apparatus as claimed in claim 10 is used in saidNMR probe.
 24. A nuclear magnetic resonance analysis apparatuscomprising: a split type superconducting coil; and a probe for an NMRinserted to the split, wherein the low temperature probe used in the NMRapparatus as claimed in claim 11 is used in said NMR probe.
 25. Anuclear magnetic resonance analysis apparatus comprising: a split typesuperconducting coil; and a probe for an NMR inserted to the split,wherein the low temperature probe used in the NMR apparatus as claimedin claim 12 is used in said NMR probe.
 26. A nuclear magnetic resonanceanalysis apparatus comprising: a split type superconducting coil; and aprobe for an NMR inserted to the split, wherein the low temperatureprobe used in the NMR apparatus as claimed in claim 13 is used in saidNMR probe.